Atrial fibrillation (AF) is the most common clinically important cardiac arrhythmia. The arrhythmia is thought to result from two mechanisms. The first is a “trigger” site proximate the heart that stimulates an abnormal electrical signal that is propagated through the atrial walls. Some researchers have concluded that paroxysmal AF is induced by triggers in and adjacent to the pulmonary veins and their connection to the atrial wall in 90% of cases.
The second mechanism is a cardiac re-entry abnormality in conduction that allows and perpetuates the abnormal conduction within the atrial wall. Based on extensive animal studies in the 1980's, it has been postulated that AF resulted from the presence of a macro re-entrant circuit in the atria. These studies and others lead to the development of the Cox maze surgical procedure for the treatment of AF in 1987. The maze procedure consists of a series of transmural incisions in the atrial wall and cryoablation lesions to prevent the development of the macro re-entry circuit. Ablation results in the destruction of some portion of the tissue in the atrial wall thereby providing a portion of ablated tissue having a high electrical resistance to prevent the undesirable propagation of abnormal electrical signals in the heart.
Many clinical studies have documented the long term success of the Cox maze procedure in the restoration of sinus rhythm and elimination of AF, the preservation of atrial function, and the elimination of stroke caused by cerebral emboli. Due to the technical demands of this procedure, it was only used in a limited number of patients and less demanding and less invasive procedures were sought. Accordingly, some prior art AF treatment techniques utilize a bipolar radio frequency clamp device to create lines of ablated tissue through transmission of radio frequency energy through the atrial wall to replace many of the incisions of the “cut and sew” maze procedure. This technique resulted in success rates similar to the Cox maze surgical procedure. With the demonstration that creation of lines of electrical block in atrial tissue could be effective in treating AF, a wide variety of energy sources and multiple surgical approaches to their application have been applied to treat AF. The ideal procedure achieves transmural lines of injury and conduction block in atrial tissue, causes no damage to neighboring structures, and is applied in a minimally invasive fashion.
Catheter ablation techniques for AF treatment applied through a transfemoral access and transeptal approach to the left atrium presently provide the least invasive technique. Catheter ablation using radio frequency energy transmission has been applied with varying success rates. Success rates of 45-85% utilizing this approach have been reported in patients without structural heart disease. However, patients with chronic AF typically receive temporary antiarrhythmic drug therapy after an ablation procedure and often require one or more cardioversions or ablations to restore sinus rhythm. These are confounding variables that may inflate the actual efficacy of these prior art catheter ablation techniques. Follow up data on these patients may vary with the means of data collection and because AF may be asymptomatic, these factors may also contribute to over estimation of success rates.
An AF treatment technique utilizing an endocardial application of unipolar radio frequency energy has been employed but is subject to two significant complications. It has proven quite difficult to determine the energy necessary to create transmural lesions in atrial tissue of varying thickness. Accordingly, perforation of the atrial wall can occur with bleeding and tamponade as well as perforation of adjacent structures, most notably the esophagus. Additionally, the application of unipolar radio frequency energy in the pulmonary veins can result in their constriction with resultant pulmonary vein stenosis and its attendant complications.
Some bipolar energy delivery systems have also been attempted in the art to provide both epicardial and endocardial therapy for AF and as such, afford some advantages over prior art techniques. With bipolar energy application, the targeted tissue receives just enough energy to produce a transmural lesion which can significantly shorten ablation time. It also decreases thermal spread and thus reduces the chances of injury to adjacent structures. Impedance changes measured utilizing various electrical sensing mechanisms at the time of ablation aid in the precise delivery of energy necessary to predictably create transmural lesions.
The premise for success of the maze procedure is complete block of electrical transmission across the line of incision or ablation. Success rates of 90-95%—similar to the “cut and sew” maze procedure—have been achieved with bipolar radio frequency ablation techniques, which require an open heart cardiac procedure in conjunction with cardiopulmonary bypass and cardiac arrest. Similar excellent results have been reported with the use of cryotherapy, again requiring an open cardiac procedure with cardiac arrest and the addition of lines of ablation as described in the “cut and sew” maze procedure. These prior art procedures, while efficacious, are undesirable due to the invasive approach required for the treatment, with its concomitant risks.
Other prior art surgical procedures to provide a less invasive approach have been utilized as well. Energy sources including radio frequency, microwave, and cryotherapy used in an open heart procedure have produced results similar to the “cut and sew” maze procedure as described above, but have been less successful or unproven in a minimally invasive approach.
A variety of clamp-type devices have been employed in a minimally invasive approach. These devices are typically positioned around the pulmonary veins thence secured tightly in place and provide electrically conductive surfaces that contact the atrial wall. The clamp devices create lesions by application of energy across two thicknesses of atrial wall, and often require as many as three of four applications to provide transmural lines of ablation.
Because the circulating blood within the left atrium acts as a heat sink, application of thermal energy, whether hot or cold, (such as cryotherapy) in an epicardial approach does not produce reliable and consistent transmural lesions in the atrial wall in a beating heart.
While reasonable success rates have been reported for paroxysmal AF with pulmonary vein encircling lesions alone, additional lines of ablation are necessary to match the success rates of the “cut and sew maze” procedure in persistent or chronic AF. While the number and pattern of lines from the original “cut and sew” maze procedure that are required to increase the success rate to 90% and above are the subject of debate, the addition of a line of ablation from the pulmonary vein encircling line to the mitral valve annulus has been shown to enhance the success rate of the procedure. Clamp devices for the delivery of bipolar radio frequency ablation can not be used to create this line and many of the other lines of the maze procedure in a minimally invasive approach.
Some prior art techniques utilize microwave energy applied through an epicardial device placed on the atrial wall in an open heart epicardial procedure with success rates of 70-90%. Minimally invasive use of microwave energy introduced through standard thoracoscopic techniques and applied in an epicardial approach have also been attempted.
Another prior art technique utilizes high frequency ultrasound ablation of AF used through an open sternotomy. Use of this technique with a mini-thoracotomy approach has also been disappointing with success rates of 50-70%.
Based on the foregoing, there is a need for an AF treatment technique and apparatus that combines a minimally invasive surgical approach with a bipolar energy delivery system for both enhanced efficacy in AF treatment and swift patient recovery.